Active matrix type liquid crystal displays employing thin-film transistors (TFTs) are used in various fields as displays for TV sets, camcorders, personal computers, personal word processors and the like because they can be made thin and light and driven with a low voltage. There is a large market for such displays.
In recent years, particularly for use in TV sets and computers, the demand for liquid crystal display devices having a wide viewing angle, usable for wide screens, has increased. In order to meet this demand, Japanese Unexamined Patent Publication No. 1994-160878 proposes, as a method for increasing the viewing angle of liquid crystal display devices, an IPS (In-Plane Switching) method in which the pixel electrode and the opposing electrode for driving the liquid crystal are formed on a single substrate where liquid crystal molecules are actuated by applying a voltage in a lateral direction. This display method is also known as the lateral electric field method or comb-like electrode method, where the liquid crystal molecules are arranged so that the major axes thereof are parallel with respect to the substrate, and therefore the liquid crystal molecules are never oriented perpendicular to the substrate. Thus, the variance in brightness when seen from various directions becomes less and this makes it possible to achieve a wide viewing angle.
A known IPS style liquid crystal display device will be described below with reference to drawings.
FIG. 11 is a plan view illustrating the structure of one pixel of an array substrate of a prior art liquid crystal display device. FIGS. 12(a) and (b) respectively show cross sections taken along lines P-P′ and Q-Q′ of FIG. 11. In FIG. 11, gate wirings 1 for feeding scanning signals and source wirings 2 for feeding image signals are disposed so as to intersect at approximately right angles. Nearby each intersection of a gate wiring 1 and a source wiring 2, a thin-film transistor (TFT) 5 having semiconductor layers is formed as a switching element. To the source wiring 2, a comb-like pixel electrode 3 is connected via the TFT 5, and a common electrode 4 functioning as a standard potential is arranged so as to mesh with the pixel electrode 3. The common electrode 4 is electrically coupled to a common wiring 8 disposed between the two gate wirings 1 and 1 in a parallel manner.
As shown in FIGS. 11 and 12, the gate wiring 1, the common electrode 4 and the common wiring 8 are formed on an array substrate 10 as a same layer. Upon this layer, the source wiring 2 and the pixel electrode 3 are formed as a same layer through an insulating layer 6a. At the intersection of the common wiring 8 and the pixel electrode 3, with the insulating layer 6a in between, a storage capacity region 109 is formed. The principal components of the wirings and the electrodes mentioned above include aluminum (Al), chromium (Cr), tantalum (Ta), molybdenum (Mo) and like metals.
On the surface of an opposing substrate 14 facing the array substrate 10, a black matrix 12 and a color filter 13 are formed. As shown by dash-dot-dot lines in FIG. 11, the black matrix 12 is arranged so as to cover the TFT 5 and the non-controlled area of the electric field generated between the gate wiring 1 or the source wiring 2 and the pixel electrode 3 or the common electrode 4. The color filter 13 is formed on the aperture of the black matrix 12, and each pixel thereof has a color layer of red, green or blue so that, in the liquid crystal display device as a whole, these three colors are repeated in an array.
Between the array substrate 10 and the opposing substrate 14, liquid crystal (not shown) is sealed in the gap held constant by beads applied on the substrate. Thus a liquid crystal display device can be obtained.
According to such a liquid crystal display device, the variance between the voltage applied to the pixel electrode 3 and that of the common electrode 4, to which standard potential is applied, generates an electric field substantially parallel to the substrate, and the electric field is applied to the liquid crystal disposed between the electrodes. By storing electric charge while the TFT 5 is in an on-status, even when the leakage of electric charge from the pixel electrode 3 occurs, the storage capacity region 109 can supply the voltage for the leaked portion and maintain the signal voltage at a certain level. This enables the liquid crystal to remain actuated.
The pixel electrode 3, the common electrode 4 and the common wiring 8 of the above-described liquid crystal display device are made of opaque metals, preventing light from passing through these areas. If the area where the pixel electrode 3 and the common wiring 8 forming storage capacity region 109 intersect is too small, the storage capacity becomes insufficiently small, resulting in flicker and crosstalk. Therefore, the storage capacity region 109 must have at least a certain minimal size; however, making the storage capacity region 109 larger leads to a wider non-light-transmitting area. Even in the area where light is transmitted, in some locations such as the gap between the source wiring and the gate wiring, and the common electrode and the pixel electrode, etc., it is not possible to control the light transmittance as much as is desired. Therefore, these locations should be covered with the black matrix 12.
Consequently, the known IPS style liquid crystal display device has drawbacks such as a low pixel aperture, i.e., unsatisfactory ratio of the effective display area to the area of pixel, leading to a panel with low luminance.